The present invention relates to gages and the like for linearly measuring non-conductive surfaces and, in particular, to the adaptation of the so-called electronic micrometer gages for such purposes.
As noted in the foregoing, presently available transducer systems which also are referred to as electronic micrometers generally require the presence of a conductive material to accomplish their purposes. The reason is that these micrometers generally operate by generating an electrical field which directly varies with the displacement distance of the generator from a conductive surface.
The present invention is concerned primarily with expanding the use of these prior art systems to the measurement of non-conducting surfaces. Such being the case, it may be helpful to consider in greater detail the particular types of systems which presently are contemplated. One of the better systems, for example, is known as the Kaman MULTI-VIT displacement measuring system produced by Kaman Sciences Corporation of Colorado Springs, Colo. The so-called MULTI-VIT system is, as suggested by the acronym, a multiple purpose variable impedance transducer system employing a series of individual non-contacting displacement measuring devices or units adapted to cover a wide range of measuring capabilities. In general the units each operate on an eddy-current loss principle achieved by an active transducer element in the form of an electrical bridge circuit activated by a 1 Mhz carrier. Magnetic flux lines eminating from the active element at the 1 Mhz rate pass into the surface being measured or sensed to produce eddy currents. If, as is required, the surface being sensed is a conductive surface, variations in the displacement of the active element from the conductive surface produce proportionate variations in the eddy current loss, these loss or impedance variations then being converted to a D. C. voltage proportional to the distance being sensed. In other words, as a conductive surface moves closer to the active element, more eddy-currents are generated and the losses within the grid circuit becomes greater. As the conductive surface moves away from the transducer, the losses become less. Because of the high frequency of the eddy-currents, they penetrate the conductive surface only a few mils depth so that displacement of conductive surfaces as thin as 0.50 mils have been successfully measured.
Such systems are most beneficial particularly in micro-measurement applications involving, as stated, the presence of the conductive material that produces the eddy-current variations. In fact, the increasing demands imposed by advancing technology on the accuracy of linear measurements have made classical measuring methods inadequate. The so-called Kaman electronic micrometer, however, have been found capable of providing the requisite accuracy. Measurements within a few micro inches resolution are obtained easily and quickly.
The disadvantages, however, lie in the fact that they are limited to use with conducting materials. For this reason, their benefits have not been available for measuring the linear accuracy of a number of important products which, for one reason or another, are manufactured from glass, plastics, or other non-conducting materials. One particular example of such a non-conducting object is the manufacture of a large number of missile radomes which are formed of a glass composition that requires,, as a final fabrication step, the grinding of the glass composition to an extremely close tolerance. In the past, the combination of glass residue from the grinding process and the close product tolerances that are required to have produced problems particularly with excessive gage wear which itself results in limited gage use before the need for recalibration. Obviously the so-called electronic micrometers would resolve these problems except for the fact that the radomes are non-conductive.
In addition to the Kaman electronic micrometers which operate on the eddy-current loss principle, there are other types of transducing micrometers which provide comparable advantages and for which the present invention also can be adapted. For example, another type of electronic micrometer operates on a capacitive principle rather than the eddy-loss principle. Here again, however, the use of the capacitative type is limited to the presence of the conductive material. In this regard, the conductive material forms one plate of a capacitor which, in turn, varies its output proportionately to the variation of its displacement from the conductive surface. Within the limits imposed by the present claims, the present invention contemplates the application of the benefits of all of these so-called non-contacting electronic micrometers to the fabrication measurements of non-conductive products. Before continuing, it perhaps should be noted that the term `transducer` has been used to describe these electronic micrometers. Obviously, the basic transducing capacity is that of converting variations in displacement distances to analog electrical outputs to provide readouts which if desired, easily can be converted to obtain a digital readout capability readout.
The objects and principle features of the invention are considered to be readily apparent in the foregoing discussion.